Credential enable/disable + descriptions, artist casing settings page, playlist compare view

Credentials page now shows a fact-checked one-line description per
service and marks Spotify/Soulseek/Navidrome as Required (no toggle --
they're load-bearing for every playlist sync). The four optional
integrations (Bandcamp, AzuraCast, Qobuz, Telegram) get a real on/off
switch: Bandcamp pauses its scheduled job (same mechanism as the Jobs
page), the other three just remove their rendered credential file, which
the scripts that read them already treat as "not configured, skip" --
no script changes needed. Disabled optional scopes also drop off the
dashboard's credential status row.

New Settings > Artist Casing page to view/add/remove entries in
artist-canonical.list without shelling in -- adding a name that already
matches case-insensitively replaces its casing in place instead of
duplicating.

Playlist detail page's status section is now two side-by-side columns:
the original Spotify tracklist on the left, and a tabbed Downloaded /
Waiting-to-download view on the right (client-side tabs via Alpine),
both with an internal scroll area so long playlists don't pull the
columns out of sync. Uses the existing status_service reconciliation
(sldl index + beets + quarantine) -- no backend changes needed there.

app/routers/credentials.py

@@ -18,10 +18,20 @@ async def credentials_index(request: Request, user: dict = Depends(require_auth)
         scope: set(credential_service.get_scope(db, scope).keys())
         for scope in credential_service.SCOPE_FIELDS
     }
+    enabled = {
+        scope: credential_service.is_scope_enabled(db, scope)
+        for scope in credential_service.SCOPE_FIELDS
+    }
     return templates.TemplateResponse(
         request,
         "credentials/index.html",
-        {"scope_fields": credential_service.SCOPE_FIELDS, "configured": configured},
+        {
+            "scope_fields": credential_service.SCOPE_FIELDS,
+            "scope_descriptions": credential_service.SCOPE_DESCRIPTIONS,
+            "core_scopes": credential_service.CORE_SCOPES,
+            "configured": configured,
+            "enabled": enabled,
+        },
     )
 
 
@@ -39,3 +49,12 @@ async def save_credentials(
     }
     credential_service.set_credentials(db, scope, values)
     return RedirectResponse(url="/settings/credentials", status_code=303)
+
+
+@router.post("/{scope}/toggle")
+async def toggle_scope(scope: str, user: dict = Depends(require_auth), db=Depends(get_db)):
+    if scope not in credential_service.OPTIONAL_SCOPES:
+        raise HTTPException(404, "unknown or non-toggleable credential scope")
+    currently_enabled = credential_service.is_scope_enabled(db, scope)
+    credential_service.set_scope_enabled(db, scope, not currently_enabled)
+    return RedirectResponse(url="/settings/credentials", status_code=303)